This invention relates to lasers which are tunable over a range of frequencies.
Lasers which can be tuned to emit light at one of a range of frequencies are useful in many applications, such as optical sensors and communications.
Refering to FIG. 1 such lasers generally comprise a laser cavity 1 containing a laser amplifying element 2 capable of laser amplification over a range of frequencies. The laser cavity 1 is defined by a fully reflective surface 3 and a semi reflective surface 4 through which laser light 5 escapes the cavity 1 and two light absorbing walls 7 and 8. In order to ensure that only laser light at one selected frequency is emitted by the laser a Littrow prism 6 is used.
The Littrow prism 6 can be rotated about an axis perpendicular to the paper. At any angular position of the Littrow prism 6 only one frequency of light will resonate in the laser cavity 1. This is because the refractive index of glass varies with frequency, with light of longer wavelengths being refracted less, so at any position of the Littrow prism 6 only one frequency will strike the reflective surface 3 at 90.degree. and retrace its path through the laser amplifying element 2. Light at all other frequencies will be deflected and eventually strike light absorbing walls 7, 8 of the laser cavity.
There are a number of problems with such a system. The first is that the laser must be tuned from one frequency to another by mechanically moving the Littrow prism, and such tuning will take a very long time compared to the rate at which the laser element can be pulsed or switched on and off. The second is that the mechanisms for mounting and rotating the Littrow prism will be vulnerable to shocks, so vibration or shocks to the laser could de-tune or even destroy it.
Another known way of tuning lasers is shown in FIG. 2. This system comprises a laser cavity 1, laser amplifying element 2, reflective surface 3, semi-reflective surface 4 and light absorbing walls 7 and 8 as before. A Bragg cell 9 is arranged to tune the laser by deflecting light of the required frequency along path 10 so that it strikes the reflective surface 3 at right angles and retraces its path back through the Bragg cell 9 and laser amplifying element 2. Light of all other frequencies 11 passes straight through the Bragg cell and is absorbed by a light absorbent surface 12.
The disadvantage of this construction is that it generally reduces the range of frequencies over which the laser will emit light, and for many types of laser amplifying element makes it impossible for laser light to be generated at all. This is for two reasons. The first is that each time light is deflected by the Bragg cell it undergoes a small frequency change and this frequency change may make laser emission by the system impossible. The second is that each time the light is deflected by the Bragg cell approximately 15 to 20 per cent of it continues straight through on each passage through the Bragg cell and is lost. The extra losses of light intensity due to this may reduce the overall gain of the laser system, that is the laser amplifying element 2, reflective surfaces 3 and 4 and Bragg cell 9, to less than unity and make it impossible for laser emission to occur.
It was the aim of this invention to produce a tunable laser system reducing these problems.